Laser-Illuminated Steppers and Scanners Lasers are frequently used as light sources in stepper and scanner systems for integrated circuits fabrication to selectively expose layers of photoresist on the silicon wafers. A mask (or reticule) is interposed between light and the wafer to allow the selective exposure. State of the art exposure systems include a relatively complex optical system to direct the laser radiation onto the wafer surface.
The amount of light required to adequately expose the photoresist during a step or scan is a product of the light intensity and the length of time that the light impinges upon the wafer. Typically, the laser light is generated in pulses at rates such as 1000 pulses per second, and the number of pulses needed to adequately expose the photoresist is determined based upon the intensity of the laser light. Prior art lithography lasers typically include a pulse energy monitor at or near the output of the laser. This monitor can be used in a feedback arrangement to control the energy of individual pulses and the integrated dose energy (total energy in a predetermined number of pulses). Pulse energy monitors may also be located within the stepper or scanner and efforts have been made to control the laser source using these monitors for feedback signals. However, these prior art feedback control systems have not proven very satisfactory and can create serious oscillations in the laser.